Field of the Invention
The present invention relates to the field of computer-based animation, in particular, to motion control of active deformable objects.
Description of the Related Art
Computer generated animation typically involves simulating the locomotion of various objects, such as characters and other virtual actors, in a virtual space. Many objects are articulated, in that such objects include a rigid internal structure that includes the equivalence of bones and joints. Accordingly, this rigid internal structure forms a virtual “skeleton” for the articulated objects. Such objects, such as humanoids and animals, may be animated via physics-based locomotion controllers that translate high-level commands, such as “walk left,” “crouch,” or “jump,” into sequences of joint torques that yield the desired behavior. Deformable objects, on the other hand, are generally passive, in that the objects have no such internal rigid structure to which joint torques may be applied. Examples of deformable objects include jelly-like creatures, walking carpets, singing candelabra, or water-pail-carrying broomsticks.
Such a passive deformable object may be animated using physics-based simulation by applying external control forces to the deformable object in order to achieve desired motion objectives. These external forces do not typically sum to zero and may have non-vanishing rotational components. As a result, these external forces may change linear and angular momenta related to the deformable object in arbitrary, non-physical ways, which may lead to unrealistic motion. For example, animating a deformable object by applying external forces may yield motion similar to that of a marionette controlled by an invisible puppeteer.
Alternatively, a deformable object may be fitted with an internal rigid structure, essentially providing a “skeleton” for the deformable object. Joint torques may then be applied to that internal structure, as is done for articulated objects. As a result, the deformable object moves in response to forces applied to the joints of the internal structure. However, the resulting motion is restricted to what is achievable by articulating the internal structure. Such motion may be substantially less than the motion available to the original deformable object that is not fitted with the internal structure.